There are known the following techniques related to a structure around the electrodes of a wiring board. One known example is a technique for inversely tapering the resist layer in cross section by forming a photosensitive resist layer on the main surface of an interposer substrate on which an electrode pad is formed and shifting the focus of an exposing process in removing unnecessary resist on the electrode.
Another known example is a technique for forming electrodes in the surface of the circuit board.
An electronic component having a ball grid array (BGA) structure is joined to a wiring board through reflow processing on ball-like terminals such as solder balls. However, this can cause the wiring board to suffer warping due to the heat at the reflow processing, resulting in a joining defect. For example, the warping of the wiring board may excessively increase the distance between the terminals of the electronic component and the electrodes of the wiring board, causing a joining failure. The warping of the wiring board can excessively can increase the distance between the terminals of the electronic component and the electrodes of the wiring board to crush the solder and spread the solder to form a solder bridge between the electrodes, causing a short circuit.
Another problem is that when the condition of the warp of the wiring board is changed by returning the temperature of the wiring board to which the electronic component is bonded after the reflow processing, the junction is continuously stressed, and the reliability of bonding is reduced.
An example of a method for reducing the above problems is to use solder having a relatively low melting point, such as bismuth (Bi)-based solder. However in this case, the reflow temperature is about 200° C., so that no great effect is given.
Instead of solder, a cold-setting electrically conductive adhesive can be used. The electrically conductive adhesive is an adhesive in which electrically conductive particles are dispersed in a thermosetting resin, such as an epoxy resin. A known example is silver (Ag) paste that contains Ag as the conductive particles. In recent years, an electrically conductive adhesive capable of bonding at about 80° C., which is sufficiently lower than the glass transfer temperature Tg of the wiring board, has been developed. The use of such an electrically conductive adhesive reduces the warping of the wiring board.
However, the bonding strength at the junction using the electrically conductive adhesive is lower than the bonding strength using solder, which makes it difficult to ensure the bonding reliability. In particular, it is not easy to check the appearance of the junction between the electrodes of a wiring board and the terminals of an electronic component having a structure in which the terminals are disposed on the bottom, such as a BGA. For that reason, it is preferable to join the electrodes of the wiring board and the terminals of the electronic component reliably and stably.
The followings are reference documents.    [Document 1] Japanese Laid-open Patent Publication No. 11-251472, and    [Document 2] Japanese Laid-open Patent Publication No. 2004-214357.